A new structural design and tailored morphology of polymer-functionalized graphene (polymer-FG) are employed to optimize composite polymer electrolytes (CPEs). The ionic transfer conditions including Li salt dissociation, amorphous content and segmental mobility are significantly improved by incorporating polymer-FG, especially that having a polymeric ionic liquid (PIL) and a polymer brush structure [PIL(TFSI)-FG_brush]. Electrical shorts are eliminated due to the presence of the functionalized polymer on reduced graphene oxide (RGO) and a minimal amount of polymer-FG in the PEO/Li⁺ polymer electrolytes (PEs). Polymer-FG with PIL brushes increases significantly the Li ion conductivity of PEO/Li⁺ PE by >2 orders of magnitude and ∼20-fold at 30 °C and 60 °C with high Li salt loading (O/Li = 8/1), respectively. Furthermore, significant improvements in mechanical properties are observed where only 0.6 wt% addition of the PIL(TFSI)-FG_brush led to more than 300% increase in the tensile strength of the PEO/Li⁺ at an O/Li ratio of 16/1. Li-ion battery performance was evaluated with the CPE containing 0.6 wt% of PIL(TFSI)-FG_brush, resulting in superior capacity and cycle performance compared to those of the PEO/Li⁺ PE. Thus, we believe, embedding minimal amounts of structurally and morphologically optimized polymer-FG nano-fillers can lead to the development of a new class of SPEs with high ionic conductivity for high performance all-solid-state Li-ion batteries.